Method of coating ceramics and quartz crucibles with material electrically transformed into a vapor phase

ABSTRACT

Quartz crucibles for the melting of silicon and ceramic substrates are coated with protective materials or metals at least in part evaporated from an electrode with which an arc is struck at low voltage and current to deposit material from the electrode on the substrate in a vacuum chamber. The electrode may be heated and the substrate may be sandblasted and preheated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my copending applicationSer. No. 494,302 filed May 13, 1983 which was a continuation-in-part ofSer. No. 358,186 filed Mar. 15, 1982 (U.S. Pat. No. 4,438,153) which, inturn, was a continuation-in-part of Ser. No. 237,670 filed Feb. 24, 1981(U.S. Pat. No. 4,351,855).

REFERENCE TO DISCLOSURE DOCUMENTS

This application also deals with subject matter which is related to thesubject matter of disclosure documents Nos. 078,337, 078,334 and078,329, all of Feb. 26, 1979, and disclosure document No. 082,283 filedJuly 5, 1979. These disclosure documents are hereby incorporated in thefile of the present case, by reference and in their entirety.

FIELD OF THE INVENTION

My present invention relates to a method of coating of a substrate witha material which is brought into the vapor phase by electrical means.More particularly, the invention relates to an improvement in the methodof and apparatus for vapor deposition of material upon a substratedescribed and claimed in the aforementioned applications with a view toincreasing the area over which coating can be effected with materialevaporated from an electrode, and of increasing the complexity of thesurfaces which can be so coated.

BACKGROUND OF THE INVENTION

As pointed out in the aforementioned copending applications thedeposition of material from a vapor phase onto a substrate is well knownin the coating art and in the field of surface transformation of asubstrate. Generally speaking, a body of the material to be transferredto the substrate is heated in the region of this substrate andtransformed first into a molten state and then into a vapor state. Thematerial thus undergoes two phase transformations, namely, thetransformation from the solid phase to the liquid phase and then fromthe liquid phase to the vapor phase.

The coating is generally effected in a vacuum and usually a relativelyhigh vacuum must be drawn to permit transfer of vapors from the sourceto the substrate.

Earlier systems may use induction heating to effect the aforementionedphase transformation.

In the aforementioned applications, I have described an improvement overthese earlier systems in which a substance, generally a metal may betransferred to a substrate also in a vacuum environment, utilizing anelectrode as the source of the material, by the process which involvesstriking a more or less stable arc between the molten material and acounterelectrode to thereby generate the vapors.

Coating in this manner can be effective to apply anticorrosion,protective, decorative, conductive, semiconductive or other coatingsupon a substrate and I am also able to form compounds on the substratebetween materials deposited and substances on the substrate or twomaterials to be deposited. For example the last mentioned of the threeapplications describes also the formation of carbides, borides,silicides, nitrides and silicon carbides.

PROCESS OF U.S. PAT. NO. 4,351,855

While the process of the last of the cross-referenced applicationscannot be considered prior art with respect to the instant invention, abrief review thereof may be advantageous since it serves as a backgroundto the instant improvement.

Thus this application discloses a method of vapor-depositing materialupon a substrate which, as indicated, utilizes an electrical arc struckbetween a pool of molten material and a counterelectrode, therebyvaporizing the material on the surface of the pool and permittingtransfer of the vaporized material in the vapor state to the substratein the vacuum chamber.

The pool of molten metal, in turn, can be formed by striking an arcbetween this counterelectrode and an electrode composed of the materialto be vapor deposited, the heat of the arc initially melting thematerial of this latter electrode to form the pool. The body of thematerial to be vaporized has a larger cross section than thearc-striking electrode so that the pool of molten material is formed inthis body and a cavity is created therein to receive this pool. Oneadvantage of this is that it eliminates the need for a support crucibleor container for the pool of molten material.

In this earlier system, moreover, I may move the counterelectrode intoand out of contact with the pool to thereby deposit some of the meltupon the counterelectrode and permitting the heat generated at theelectrode tip to vaporize at least in part the material transferred toit and thus in part generate the vapors which are to be transferred tothe substrate.

In the system described in this application, the initial vacuum isgenerated to a reduced pressure of the order of 10⁻⁶ torr while theoperating pressure is at a maximum of 10⁻⁵ torr and effective resultswere found requiring 100 to 250 amperes of electric current flow throughthe arc across a voltage of 70 to 120 volts, direct current. Theselevels of vacuum are generally higher than those which were usedtheretofore and the currents used were likewise considerably greater.The deposition speeds were generally 0.1 to 0.3 grams per minute.

That system (as is the case with the present system) could be used forgenerating silica, silicide or carbide coatings upon a substrate, forproducing silicides by the reaction of silicon with a substrate materialupon vapor deposition of the silicon upon the substrate, or for coatingsubstrates with practically any desired metal or alloy to formprotective coatings or coatings for other purposes.

The use of the system has been found to be widespread and metallurgical,chemical, electrical, electronic, ultravacuum, optical, rocketry,aerospace and nuclear industrial use can be found for the products. Themethod has been found to be especially effective for generatingmirror-like coatings, producing reflectors, for applying anticorrosioncoatings and films, for products of flat and other configurations, andfor producing multilayer coatings in films for semiconductor components,high ohmic resistors and wherever surface modification of a substrate isrequired. Metal nitride coatings were formed when nitrogen was admittedto the evacuated space which then had its vacuum stabilized at about2.5×10⁻³ torr. The vaporized electrode material combines with nitrogenand the corresponding nitrides are deposited.

The two-electrode system can be used to form compounds in situ frommaterials of both electrodes. One electrode can be a metal electrodecomposed of high purity, titanium, tungsten, aluminum or copper whilethe other electrode could be a silicide, boride or carbide (or silicon,boron or carbon) so that the deposit is a silicide, boride or carbide ofthe high purity metal. When one electrode is graphite or carbide and theother is a silicide, silicon carbide is formed in the arc and isdeposited. When both electrodes are silicides or borides, the coating isa silicide or boride.

A particular problem has been encountered with respect to the quartzcrucibles utilized in the production of silicon wafers for semiconductorpurposes. Such crucibles in which the elemental silicon is melted,generally are composed of quartz and are received in a supporting carbonjacket within an induction coil which is utilized to melt the elementalsilicon. A monocrystalline silicon seed can then be lowered into thesilicon melt of the crucible and raised slowly to draw a silicon barwhich is controlledly cooled so that the monocrystalline product canthen be cut into wafers. Such crucibles are, for the melting of theelemental silicon and during the drawing process brought to and held formany hours at an elevated temperature close to their softening point andat a temperature at which attack by the molten silicon can occur. Thiscauses deterioration of the crucible and threatens the introduction ofundesired impurities in the wafers which are produced.

Hence the semiconductor field has long sought a method of protectingsuch crucibles which will increase their life.

Another problem is the application of metals to ceramics. While as manymetals can be coated onto ceramics, the refractory metals such astungsten and titanium have been applied heretofore in a manner which isless than satisfactory and practically in all cases adhesion problemsare encountered by the methods used heretofore.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to extend theprinciples set forth in the aforementioned applications and thus toprovide an improved method of deposition upon a substrate wherebydisadvantages of prior art techniques are avoided.

Another object of this invention is to provide a method for the vapordeposition of material on large-area and/or complex configurationsubstrates at relatively low-energy cost and with improved uniformity.

It is also an object of the invention to provide a method for thehigh-speed coating of complex and/or large-area surfaces.

Yet another object of the invention is to provide an improved method ofprotecting quartz crucibles of the type utilized in the semiconductorfield so as to increase the life thereof.

Still another object of this invention is to provide an improved methodof applying metal coatings to ceramics whereby the poor adhesionproblems characterizing prior art systems are avoided.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained in accordance with this invention, in a method of vapordeposition which generally utilizes the principles set forth above butwhich is based upon my discovery that especially large-area surfacedeposits can be formed by juxtaposing an elongated electrode of thedepositing material, laterally with the surface of the substrate to becoated over a substantial portion of the length of the electrode in avacuum, and striking an arc between one end of this electrode and acounterelectrode such that the arc current should be between 50 and 90amperes with a voltage applied across the electrodes of 30 to 60 volts.

Surprisingly, once the arc is struck as the two electrodes areseparated, the arc, a portion of the arc or a heating effect generatedby the arc appears to spiral around the long electrode and causevaporization of the material of the electrode in a generally helical orspiral pattern progressively moving away from the counterelectrode.

It is indeed a remarkable surprise that the arc is not confined to thespace between the two electrodes but rather has a component or an effectwhich spirals away from the counterelectrode toward a region of thelength of the long electrode which is further removed from thecounterelectrode in spite of the fact that the greatest conductivitywould appear to lie in a line directly between the two electrodes wherethe major portion of the arc appears to be confined. This effect ismanifest in the fact the long electrode, i.e. the deposition electrode,while originally of uniform cross section, develops a taper toward thecounterelectrode and coating from the blank of the deposition electrodeonto the substrate can be observed at considerable distance from thearc's striking face of the deposition electrode.

In fact, the effect appears to survive for a brief period followingextinction of the original arc and hence I prefer to periodicallycontact and separate the electrodes to generate the arc and then allowextinction thereof.

According to a feature of the invention, means is provided at an end ofthe electrode of the material to be deposited, remote from thearc-striking electrode to control the temperature of thematerial-supplying electrode, generally to maintain it in the range of800° F. to 1000° F., the speed under the lower voltage, lower currentand temperature conditions of the present invention, at which thematerial evaporates from the material-supplying electrode, can beincreased by 1.5 to 2.0 times the speed of evaporation of the earlierapplications. Practically all metals, alloys, carbides and silicides canbe used in making the material-supplying electrode. In addition tometals and other alloys, carbides, borides, silicides and nitrides canbe deposited on the substrate.

While I do not fully understand why the rate of evaporation of thematerial to be deposited increases with the lower energy utilization ofthe present invention, it is possible that the migration of the arc mayspread the otherwise pooled molten phase over a wider area of thematerial-supplying electrode to allow, in effect, evaporation of themolten metal in thin film form.

According to another feature of the invention, the principles set forthabove are utilized in the application of metal coating to syntheticresins, the synthetic resins in the form of cabinets or housings forelectronic components. I have found, most surprisingly, that since thesubstrate is unaffected by the large area coatings which can be appliedin accordance with the invention, the invention is highly advantageouswhen utilized to coat the interiors of synthetic resin cabinets orhousings which may be utilized for electronic components, the coatingforming an electromagnetic shield.

I have also found, most surprisingly, that the invention is highlyeffective in applying pure silicon coatings, or other protectivecoatings, e.g. silicon carbide, silicon nitride or boron nitride, to theinterior surfaces of the quartz crucibles hitherto utilized for themelting of silicon in the production of the monocrystalline bars to bedrawn from the molten silicon in the semiconductor field. Furthermore,the invention may be utilized in the application of metal coatings toceramics, with improved adhesion, even when the applied metals arenickel, tungsten, titanium, tantalum and like refractory metals whichhave been difficult to apply heretofore to ceramic substrates.Practically any ceramic substrate may be utilized for the purpose of theinvention and in the case of the quartz crucibles and the ceramicsubstrates, it is preferred to subject the surface adapted to receivethe coating to a sandblasting or other blast-roughening procedure. Theterm "sandblasting" is here utilized to describe the entrainment ofabrasive particulates against the surface, the abrasive particulatesbeing generally metal particles, silicon carbide, silicon nitride,diamond dust, iron oxide, silicon dioxide or any other material capableof surface roughening. The entraining gas can be air or any otheravailable gas. In both cases, in addition, the substrate may bepreheated within the vacuum chamber or prior to introduction into thevacuum chamber to a temperature less than the melting point of themetal. The preheating temperature should be at least several hundreddegrees, however.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will become more readily apparent from the followingdescription, reference being made to the accompanying drawing in which:

FIG. 1 is a diagram in elevational view illustrating an apparatus forcarrying out vapor deposition in accordance with an embodiment of thepresent invention;

FIG. 2 is a similar view of another apparatus wherein, however, thevapor deposited material is collected on a vertically reciprocalelectrode;

FIG. 3 is a vertical section, also in diagrammatic form, illustrating anapparatus for depositing material upon a substrate disposed below thepool of metal;

FIG. 4 is a view similar to FIG. 3 illustrating another embodiment ofthe invention;

FIG. 5 is an axial cross-sectional view of another apparatus fordepositing material upon a substrate according to this invention;

FIG. 6 is an axial cross-sectional view of a highly compact portableapparatus for carrying out the method of the invention;

FIG. 7 is a diagrammatic cross sectional view of another apparatus forcarrying out the present invention;

FIG. 8 is a diagrammatic section illustrating the application of theinvention to the coating of a quartz crucible for use in the productionof semiconductor wafers; and

FIG. 9 is a view of still another device diagrammatically illustratingthe application of large area coatings to ceramic substrates accordingto the invention.

SPECIFIC DESCRIPTION

In FIG. 1 I have shown a system utilizing a simple arc method inaccordance with the present invention for obtaining mirror-likeprotective coatings upon substrates or for evaporating various metals ormetal alloys, including heat-resistant and refractory metals, to applycoatings thereof to the substrate.

As is apparent from FIG. 1, the basic apparatus can include a vacuumchamber, not shown, which can be similar to the vacuum chamber of FIG. 6and in which a metal electrode 1 can be fed by an electrode feeder 7toward an electrode body 2 to form the pool 3 of molten metal with whichthe arc 4 is struck.

The electrode body 2 is held in a fixture or holder 5 and thedirect-current source applies the arc current across the electrode 1 andthe body 2 via a conventional arc stabilizing circuit represented at 8.

It has been found to be advantageous to provide the relatively smallcross-section electrode 1 with a thermal regulator 6 tending to preventoverheating of this electrode.

Since the cross section of body 2 is substantially larger than that ofthe electrode 1, the pool 3 lies in a concave recess formed in situ inthe body 2.

EXAMPLE 1

The apparatus of FIG. 1, utilizing electrodes 1 and 2 of titanium,aluminum, tungsten, tantalum or copper, strikes an arc at a temperatureof 5000° to 7000° F. to generate vapor of the metal of the pool 3 whichtraverses the distance of 10 to 15 cm to the substrate 10 and form acoating of the metal thereon. The pool 3 can be formed by a mixture ofmetal contributed by the electrodes 1 and 2, thereby depositing an alloyof the metals of the two electrodes upon the substrate. Preferably theelectrode is composed of titanium while the molten metal predominantlyconsists of aluminum, tungsten, tantalum or copper.

The apparatus of FIG. 1, without substantial modification, can beutilized in a noncrucible method of generating protective coatings ofcarbides, for producing silicide coatings on the substrate or forforming carbide or silicide and even silicon carbide layers upon thesubstrate. To deposit silicon carbide-tungsten carbide layers upon thesubstrate, electrode 2 is composed of graphite and electrode 1 oftungsten silicide. The vacuum is initially drawn to 10⁻⁶ torr andmaintained at 10⁻⁵ torr or lower. The indirect current arc-generatingvoltage is 100 volts and the arc current 150 amperes. The deposit formsat a rate of about 0.2 grams per minute.

In this case, the apparatus of FIG. 1 is used, again in the usual vacuumchamber, although the electrode 1 can be composed of silicon or carbonwhile the electrode 2 is composed of a metal whose silicide or carbideis to be formed or, in the case of a deposit of silicon upon thesubstrate, can also consist of silicon.

For example, when a silicion carbide deposit upon the substrate 10 isdesired, the electrode 1 may consist of silicon while the electrode 2 isa carbon block in which a pool 3 of silicon and solubilized carbon isreceived.

The vapors are transferred to the substrate and deposited in a siliconcarbide layer thereon. The substrate may be titanium and the depositformed on the substrate may be a mixture of titanium silicide andtitanium carbide.

Alternatively, when the electrode 1 is composed of silicon or carbon,and the electrode body 2 is composed of titanium, titanium carbide orsilicide can be deposited on a substrate of a different composition.

When a slight oxidizing atmosphere is provided in the evacuated chamber,silicon dioxide deposits are formed on the substrate.

Obviously the apparatus of FIG. 1 is particularly effective in theproduction of semiconductors.

The thermoregulator 6 may be duplicated along the length of theelectrode 1 and additional thermoregulators may be provided for theelectrode body 2 to prevent overheating thereof.

When either the electrode 1 or the body 2 is composed of silicon and theother is composed of carbon, silicon carbide is generated by thereaction and deposits in a higher purity than that of the originalsilica and carbon.

When both of the electrodes are composed of silicon, high density silicaand silicon deposits can be obtained as is particularly desirable forthe coating of semiconductors.

The apparatus of FIG. 2 is generally similar to that of FIG. 1 butoperates under somewhat different principles, the evaporation beingeffected at least in part from the wetted upper electrode 101.

In this figure, elements which correspond to those of FIG. 1 utilizesimilar reference numerals differing in the hundreds position.

In FIG. 2, the electrode feeder 107 is coupled with a verticalreciprocator 112 which imparts a reciprocation to the electrode 101 inthe direction of the arrow 114 so as to periodically plunge the tip ofthe electrode 101 into the pool 103 of the molten metal formed in theelectrode body 102.

Upon rising from this pool to restrike the arc 104, the coating 113 ofmolten metal upon the electrode 101 is evaporated and the deposit isformed upon the substrate 110.

The electrode body 102 is shown in the holder 105 and the arc currentsupply is formed by the direct current source 109 and the stabilizer 108in the manner described, the electrode 101 being provided with thethermoregulator 106.

This system has been found to be particularly effective, in amodification of the foregoing example, when the electrode 101 iscomposed of titanium and the pool 103 is formed of aluminum.

In FIG. 3 I have shown an embodiment of the invention in which the vaporis deposited upon a substrate 210 disposed below a crucible 217 in theform of a upwardly open ring containing the molten metal 203, thecrucible being mounted in a holder or frame 205.

Here the upper electrode 201 is in the form of a spherical segment whichfunctions as a reflector so that, when an arc 204 is struck between theelectrode 201 and the melt in the crucible 217, the vapors pass upwardlyas represented by the arrows 219 and are reflected downwardly to focusupon the substrate 210 as represented by the arrows 218.

The direct current source 209 is here connected across the electrode 201and the crucible 217 via the arc stabilizer 208 and the upper electrode201, mounted on the rod 216, is vertically positioned by the feeder 207and horizontally positioned by an auxiliary mechanism 215 which adjuststhe position of the electrode 201 over the evaporating metal.

In this embodiment, the electrode 201 can be composed of titanium,molybdenum or tungsten while the molten metal can be composed ofaluminum or copper and the crucible 217 of graphite.

In FIG. 4 I have shown another embodiment of the invention in which thevapors flow downwardly to deposit upon the substrate 310.

In this case, the upwardly open crucible 317 containing the molten metal303 can be supplied with additional molten metal from a ladle or othersources represented at 322 or with solid metal which is melted in thecrucible 317. The latter can be heated by auxiliary means such as aninductive heater 323 and is supported in a holder 305.

The bottom of the crucible 317 is formed with apertures 321 at whichdroplets of the molten metal appear, these droplets being vaporized bythe arc 304 struck between the electrode 301 and the bottom of thecrucible 317.

The temperature in the region of the arc can be controlled by anauxiliary inductive means 324 and the electrode 301 can be cooled asrepresented by the cooling element 306.

Electrode 301 is fed toward the crucible 317 by the electrode holder 307and the arc is maintained by an arc stabilizer 308 connected to thedirect current source 309.

In this embodiment, the molten metal may be copper.

In place of the auxiliary device 324, a substrate to be coated may beprovided at this location, e.g. in the form of a titanium ring, whichcan collect the vapor in the form of a coating.

The embodiment of FIG. 5 evaporates the molten metal as it is formed ina closed space, the vapors being discharged through apertures 425 on thesubstrate 410.

In this case, the pool of liquid is formed by melting the electrode 402supported by the holder 405 by feeding the counter electrode 401 via theelectrode feeder 407 through a central bore 426 in the electrode 402,the electrode 401 passing through an insulating sleeve 427 forming aguide. A temperature regulator 406 is provided coaxially around the twoelectrodes adjacent the arc 404 to prevent overheating in the regionahead of the apertures 425. The deposit is formed on the substrate 410.

The current is supplied between the electrodes through the arcstabilizer 408 and the direct current source 409 in the manner describedpreviously.

FIG. 6 shows a portable voltaic arc device for depositing reflective,anticorrosive, protective and semiconductor type metal, silicide andcarbide coatings using the principles described previously.

This apparatus comprises a vacuum chamber 500 which is formed at itsupper end with a handle 530 enabling the portable unit to be readilytransported.

Within this chamber, there is provided a hollow sphere 517, the lowerpart of which forms a crucible for the molten metal 503, coatedinternally with a high-temperature heat resistant (refractory) materialsuch as aluminum oxide.

The upper portion of this sphere is coated at 531 with a reflectivelayer concentrating the heat reflected from the bath back onto thelatter.

An arc 504 is struck between an electrode 501 and the bath 503, theelectrode being fed by the unit 507 toward the bath as the electrodematerial is consumed.

Additional metal, e.g. in solid form, is fed to the bath as a rod 532which also is connected to the feeder 533' so that as the bath isconsumed, additional metal is supplied thereto.

The electrode 501 and the bath 503 are connected to opposite terminalsof an arc stabilizer and a direct current source in the mannerpreviously described.

A tubular electrode 502 surrounds the rod 532.

The lower part of the chamber 500 is provided with an airpump asrepresented at 533, the latter evacuating the chamber containing thehollow sphere 517 and, via a vacuum hose 534, via a valve 535, anadapter 536 of outwardly divergent configuration which can be connectedto a lateral aperture 525 of the hollow sphere 517.

The chamber 500 can be formed with a heating coil 537 to preventundesired condensation of vapor thereon.

Between the aperture 525 and the adapter 536 there is provided a vacuumlock 538 and a mounting arrangement 539 for holding a variety ofadapters of different shapes and sizes.

The adapter 536 is also formed with a vacuum gasket 540 whereby theadapter can bear against the substrate 510 to be coated.

The portable unit shown in FIG. 6 is carried to the location of thesubstrate 510 to be coated and the appropriate adapter 536 is mounted onthe fitting 539 and the gasket 540 pressed against the surface 510 to becoated. The arc current is supplied and the system is evacuated by theair pump 533, thereby melting the metal and forming the bath 503 withinthe hollow sphere. The gate 538 is then opened and the vapors permittedto pass onto the substrate 510 at least in part by pressure differentialas controlled by the valve 535 maintained between the interior of thesphere 517 and the adapter 536.

Practically any product at any site can be coated and the use of avariety of adapters of different shapes and sizes enables coating ofeven intricate bodies without moving them from the area in which theyare to be used. The device can be collapsible so as to be used toprovide coatings inside ducts and the like.

The apparatus shown in the drawing, without the adapter 536, can be usedas a propellant for individuals or equipment in space.

Upon generation of the arc, one need only open the gate 538 to dischargea stream through the aperture 525 and effect propulsion in the oppositedirection. The vacuum in space provides a natural vacuum for the deviceand no air pump 533 is then required. Practically any waste found inspace applications can be utilized in the vessel 517 to generate suchpropulsion.

In FIG. 7 I have shown an embodiment of the invention which combinesfeatures previously described and concepts developed above.

In this system, which can be used to deposit a coating 610' on the innersurface 610a of a tube 610, forming a substrate, of complex shape, amaterial-supplying electrode 602 of corresponding shape is mountedcentrally of the tube on a support 602a and is provided with aninduction heating coil 606a of a temperature controller 606 which canhave a thermocouple 606b or a like temperature sensor responsive to thetemperature of the material-supplying electrode 602 for maintaining thetemperature of the latter constant in the range of 800° to 1000° F. byconventional feedback control circuitry.

As in the previous embodiment, the substrate and the source of thematerial to be deposited on the substrate are enclosed in a vacuumchamber 600 which can be evacuated to 10⁻⁶ torr so that vapor depositioncan be effected at a pressure of 10⁻⁵ torr.

The end of the material-supplying electrode 602 is provided with anarc-striking electrode 601 which can be reciprocated toward and awayfrom the electrode 602 by an electrically controlled reciprocating drive607. The latter can be operated in response to a zero current detector607a so that when the arc current decays completely, the electrode 601is displaced to the left into contact with the end 602a of the electrode602 and is then withdrawn to reestablish an arc. The arc current isprovided by a pulsating direct current source 609 across which an arcstabilizer 608 the parameters of the arc current and arc voltage areadjusted within the range of 50 to 90 amperes and 30 to 60 volts bythese circuit elements.

In practice, utilizing the system illustrated once the arc is struck,the arc itself, an evaporation effect or some other electromagneticphenomenon appears to progress as represented by the arrow A generallyhelical and spiral where arc-striking location and vapor depositiontakes place over the entire length of the material-supply electrode 602which is subjected to this phenomenon, i.e. over the length at which thephenomenon is effective until the arc decays.

The material loss from the electrode 602 gradually transforms it into atapered shape as represented by the dot-dash lines as 602b in FIG. 7.

The fact that the taper results in a recession of the electrode from thesubstrate does not create any problem of significance because thegreatest deposit is at the region of greatest recession andconsequently, the ultimate coating as it progresses along the substrateis highly uniform.

The system of the invention is especially useful in coatingtemperature-sensitive materials with very small thicknesses of coatingmaterial since the coating is especially rapid and it is possible tocarry out the deposition without significantly heating up the substrate.

EXAMPLE 2

A copper electrode 602 of the shape shown is provided in a substratetube with an initial spacing of electrode 602 from the substrate ofabout 10 cm. The electrode is maintained at a temperature of 900° F. andan arc is struck in the manner previously described at one end. The arccurrent is about 70 amperes and the voltage applied after the electrode601 is withdrawn to form the arc is about 40 volts. The speed ofevaporation from electrode 602 under these conditions exceeds the speedof evaporation in Example 1.

In FIG. 8 I have shown an arrangement for applying a silicon coating710' to a quartz crucible 710 of the type utilized for the melting ofsilicon and from the melt of which a monocrystalline bar of silicon canbe drawn for subsequent slicing into silicon wafers and use in thesemiconductor industry. According to the invention, the interiorsurfaces of the crucible are sandblasted and the crucible is preheated,e.g. to a temperature of 200° to 600° C. before the crucible is placedin the vacuum chamber. A pair of silicon electrodes 701 and 702 arejuxtaposed with one another within the crucible and, utilizing theelectrode reciprocating means of the type shown at 607, the electrodesare brought together and then separated as represented by the arrows707a and 707b so they touch and then are drawn apart to stroke the arc.The power supply which can also be of the type described is representedat 709. The arc current can again be 50 to 90 amperes and the arcvoltage some 30 to 60 volts. a substantially uniform highly adherentsilicon coating can be obtained, especially when the silicon isinitially heated, e.g. by means similar to that utilized in theembodiment of FIG. 7.

When a nitrogen atmosphere is released in the region of the arc, thedeposit is of the silicon nitride Si₃ N₄. When one of the electrodes iscomposed of carbon, a silicon carbide deposit is formed. The same systemcan be used for coating any substrate with pure Si or one of the othercoatings mentioned. After the initial arc is produced the electrodes canbe cooled.

In FIG. 9 I have shown a system for the large area coating of a ceramicsubstrate 801 which prior to introduction into the vacuum chamber can beinitially preheated, after its coating-receiving surface has beensandblasted, by the movement of a burner 820 along the underside of thesubstrate. The electrode 801 can be composed of a refractory metal ornickel and via the actuator 807 is urged into contact with and withdrawnfrom contact from the counterelectrode 802 which may also be composed ofthe same metal.

The power supply has been represented at 809. The electrodes are heremounted on a track 821 and are moved along the substrate so that as thearc is repeatedly struck and the arc travels along the electrode 801 inthe vacuum chamber receiving the entire assembly, the entire surface ofthe substrate is coated utilizing the principles described in FIG. 7.

EXAMPLE 3

Utilizing an apparatus operating with the principles shown in FIG. 9, analuminum oxide plate is coated to a thickness of 1 to 2 mils tungstenutilizing tungsten electrodes. The arc current is 50 amperes and the arcvoltage 40 volts for a maximum electrode spacing of approximately 4 mm.The electrode diameter was about 1 cm. The tungsten coating was highlyadherent to the alumina plate.

I claim:
 1. A method of coating a quartz crucible for use in the meltingof silicon which comprises the steps of:juxtaposing a pair ofelectrodes, composed of at least one component of a material adapted tocoat said crucible, with an interior surface of said crucible;evacuating the space in which said electrodes are juxtaposed with saidsurface to a pressure of at most 10⁻⁵ torr and maintaining the pressurein said space substantially no higher than 10⁻⁵ torr during deposition;and striking an electrical arc between said electrodes at one end ofeach of said electrodes at a voltage of substantially 30 to 60 volts andwith a current of substantially 50 to 90 amperes by intermittentlybringing said electrodes into contact with one another and separatingthem, thereby depositing material evaporated from said electrodessubstantially uniformly over said interior surface of said crucible. 2.The method defined in claim 1 wherein at least one of said electrodes iscomposed of silicon, further comprising the step of controlling thetemperature of said one of said electrodes to maintain said temperaturein the range of substantially 800° F. to 1000° F.
 3. The method definedin claim 1 further comprising the step of roughening said surface byblasting particles thereagainst.
 4. The method defined in claim 3,further comprising the step of preheating said crucible before strikingsaid arc.
 5. A method of coating a ceramic with a metal, comprising thesteps of:receiving a surface of a ceramic substrate by subjecting it toblasting with particles; preheating said substrate to a temperature ofat least 200° C. but less than the melting point of a metal to beapplied thereto; juxtaposing said surface of said substrarte with anelectrode composed of said metal; evacuating the space in which saidelectrode is juxtaposed with said substrate to at most 10⁻⁵ torr andmaintaining the pressure in said space substantially no higher than 10⁻⁵torr; and striking an arc with said electrode by intermittentlyattacking same with another electrode while applying a voltage ofsubstantially 30 to 60 volts across said electrode and passing a currentof substantially 50 to 90 amperes through said electrodes to evaporatethe electrode composed of said metal and deposit said metal on saidsurface.
 6. The method defined in claim 5 wherein said metal is composedof nickel, copper, tungsten, titanium or tantalum.
 7. The method definedin claim 5 wherein said ceramic is a body of alumina.